In wireless transceiver, the mixer is widely used to be a frequency conversion element. FIG. 1 shows a direct conversion receiver 10, comprising an antenna 11, a Low-Noise Amplifier (LNA) 12, a mixer 13, a Local Oscillator (LO) 14, a Low Pass Filter (LPF) 15 and an amplifier 16. A Radio Frequency (RF) signal is received by the antenna 11, and amplified by the LNA 12, then down-converted to a baseband signal directly by the mixer 13. Thereafter, the baseband signal is filtered by LPF 15 and amplified by the amplifier 16, then sent to the backend circuit for Analog-Digital Conversion (not shown). The LO 14 generates an oscillation signal, of which the frequency is fLO, and the frequency fLO is a RF carrier frequency supplied to the mixer 13 for converting the RF signal to the baseband signal directly. The direct conversion receiver 10 need not convert the RF signal to an Intermediate Frequency (IF) signal and then convert the IF signal to the baseband signal, so it is also referred to as a “Zero-IF Receiver.” Because there is no IF conversion, the direct conversion receiver 10 has two important advantages besides saving a set of mixers. One advantage is that there is no image signal interference, and thus an Image-Rejection Filter is not needed. The other advantage is that the Low Pass Filter (LPF) 15 and the amplifier 16 may be integrated into a single IC and replace an external Surface Acoustic Wave filter (SAW filter) required by a traditional receiver. Therefore, the direct conversion receiver 10 has several advantages including higher integration, lower complexity and lower cost.
But the direct conversion receiver 10 also has some disadvantages, such as second-order inter-modulation distortion, DC offset, flick noise, etc. The second-order inter-modulation distortion is mainly caused by feedtrough, as shown in FIG. 2A. Two strong interference signals are very close to the receiving channel, and they are all in the range of the band-pass filter. An interference signal around the DC is generated when the two interference signals pass through the low noise amplifier (LNA) 211. Then, the interference signal around the DC passes through a mixer 212. If the mixer 212 is an ideal mixer, the interference signal around the DC will be converted into a higher spectrum by the mixer 212. But an actual mixer has feedtrough, so that the output of the mixer 212 includes an interference signal around the DC. As shown in FIG. 2B, the LO leakage is inputted to the LNA 221 and the mixer 222 because isolation between the components is not perfect. Therefore, a DC offset will be generated and interfere with the baseband signal. In addition, the non-linear characteristic and low frequency conversion gain of the transconductor circuit in a mixer also strengthen the IM2 effect. Therefore, it is an important issue to cancel IM2 when designing a direct conversion receiver. The present disclosure provides a negative feedback circuit for adjusting the input signal of the transconductor circuit in a mixer to overcome this problem.
FIG. 3 shows a Gilbert mixer circuit in accordance with the prior art. The Gilbert mixer 30 comprises a transconductor circuit 31, a switch quad circuit 32 and a load circuit 33. The load circuit 33 includes two parallel connected resistors RC1, RC2. More specifically, the first ends of the resistor RC1 and the resistor RC2 are coupled to a voltage source Vcc, and the second ends of the resistor RC1 and the resistor RC2 are respectively coupled to the differential out ends of the switch quad circuit 32. The switch quad circuit 32 includes NPN bipolar junction transistors (BJT) Q3, Q4, Q5, Q6. Specifically, the collector of the BJT Q3 and the collector of the BJT Q5 are coupled to the second end of the resistor RC1, and the collector of the BJT Q4 and the collector of the BJT Q6 are coupled to the second end of the resistor RC2. Furthermore, the base end of the BJT Q3 is coupled to the base end of the BJT Q6, and the base end of the BJT Q4 is coupled to the base end of the BJT Q5. Differential LO signals fLO are respectively inputted to the base ends of the BJT Q3 and the BJT Q4. Moreover, the emitter end of the BJT Q3 is coupled to the emitter end of the BJT Q4 to form a first current path, and the emitter end of the BJT Q5 is coupled to the emitter end of the BJT Q6 to form a second current path.
The transconductor circuit 31 includes NPN BJTs Q1, Q2. Specifically, the collector end of the BJT Q1 is coupled to the first current path of the switch quad circuit 32, and the collector end of the BJT Q2 is coupled to the second current circuit of the switch quad circuit 32. The base ends of the BJTs Q1 and Q2 respectively receive the voltage signal Vin+ and Vin−. Furthermore, the emitter ends of the BJTs Q1 and Q2 are respectively coupled to first ends of the resistors RE1 and RE2. The second or other ends of the resistors RE1 and RE2 are coupled to ground.
The transconductor circuit 31 transforms the input voltage Vin (i.e., differential input signals Vin+ and Vin−) to the current signal Ib. The current signal Ib is transformed to a frequency-converted current signal by the first current path and the second current path of the switch quad circuit 32 controlled by the local oscillation signal fLO. Then, the frequency-converted current signal is transformed to an output voltage at the out end of the circuit.
Because the transconductor circuit 31 consists of NPN BJTs Q1, Q2, the relationship curve between the voltage and the current is an exponential curve, and not a linear curve. Therefore, there will be IM2 current generated in the mixer, and external voltages will appear at the emitters of the BJTs Q1 and Q2, which are expressed as follows:VE1—IM2=1/α1*IC1—IM2*RE1 VE2—IM2=1/α1*IC2—IM2*RE2 
Wherein, α1 represents the common-base current gain of the BJT Q1; α2 represents the common-base current gain of the BJT Q2.
IM2 distortion causes serious interference to the original signal in the mixer. Therefore, it is an important issue to cancel the IM2 in a direct conversion receiver, and the present disclosure provides a negative feedback circuit for adjusting the input signal of the transconductor circuit in a mixer to resolve this problem.